While we're on the subject, here's another:

"But it's a nice thing, surely, to be familiar with a lot of subjects." Well, in that case let us retain just as much of them as we need. Would you consider a person open to criticism for putting superfluous objects on the same level as really useful ones by arranging on display in his house a whole array of costly articles, but not for cluttering himself up with a lot of superfluous furniture in the way of learning? To want to know more than is sufficient is a form of intemperance. Apart from which this kind of obsession with the liberal arts turns people into pedantic, irritating, tactless, self-satisfied bores, not learning what they need simply because they spend their time learning things they will never need.

-Seneca, Letter 88